Low-alloy high-tensile strength steel

ABSTRACT

A low-alloy high-tensile strength steel having an excellent toughness at a low temperature in the state as hot-rolled, the basic composition of said steel being developed by adding 2.3 to 4.0% Mn and 0.03 to 0.15% NR to a very low-carbon steel.

United States Patent Gohda et al.

[ 1 Apr. 30, 1974 LOW-ALLOY l-llGH-TENSILE STRENGTH STEEL Inventors: Susumu Gohda; l-lirokichi Higashiyama; Katuzi Nakamura, all of Kitayushu, Japan Assignee: Nippon Steel Corporation, Tokyo,

Japan Filed: Mar. 21, 1972 Appl. No.1 236,742

Related US. Application Data Continuation-impart of Ser. No. 856,342, Sept. 9, 1969, abandoned.

Foreign Application Priority Data [58] Field of Search 75/124 [56] References Cited UNITED STATES PATENTS 3,562,028 2/1971 Heitmann 75/124 2,140,237 12/1938 Leitner 75/124 3,178,279 4/1965 Nakamura 75/124 3,681,057 8/1972 Kawakami 75/124 Primary ExaminerHyland Bizot Attorney, Agent, or FirmWenderoth, Lind & Ponack [57] ABSTRACT A low-alloy high-tensile strength steel having an excellent toughness at a low temperature in the state as hotrolled, the basic composition of said steel being developed by adding 2.3 to 4.0% Mn and 0.03 to 0.15% NR to a very low-carbon steel.

4 Claims, 2 Drawing Figures vEo (kg m/cm TENSILE STRENGTH (kg-/cm 1 LOW-ALLOY HIGH-TENSILE STRENGTH STEEL This is a continuation-in-part of co-pending application Ser. No. 856,342 filed Sept. 9, 1969 now abandoned.

This invention relates to an Mn-Nb steel made by .adding 2.3 to 4.0% Mn to a low-carbon steel to obtain a low-alloy high-tensile strength steel having an excellent toughness at a low temperature.

Heretofore, for a low-alloy high-tensile strength steel there have been developed many steels such as Si-Mn, Mn-Nb and Si-Mn-V steels or steels prepared by the composite addition of Ni, Cr, Mo and V to these steels, and they have been used as hot-rolled or thereafter subjected to heat treatments comprising normalizing and- /or tempering.

Tlowe ver, as showniri l aldle 1, these lowalloy hightensile strength steels do not have sufficient low temperature toughness when used as rolled. Therefore, they have been usually used after being heat-treated.

The present invention has as an object to provide a new type of low-alloy high-tensile strength steel which has a fine and uniformly-sized upper bainite structure and is very high in low temperature toughness by improving such conventional low-alloy high-tensile strength steels as are mentioned above and to provide particularly a steel which is high in both strength and low temperature toughness as rolled and tempered).

(as not quenched When C is contained in a range of 0.05 to 0.08 percent and Mn is contained in an amount of more than 2.3 percent with a coexistence of a small amount of Nb solid-dissolved by heating prior to hot-rolling, the transformation of forming proeutectoid ferrite and pearlite is remarkably retarded and an upper bainite is easily formed. That is, a steel made only of an upper bainite structure containing neither proeutectoid ferrite nor pearlite can be obtained in a cooling process after the hot-rolling with a considerably wide range of cooling rate. Further, Nb once solid-dissolved in austenite has the effect of remarkably suppressing the recrystallization of austenite during or after the hotrolling in the case of the steel composition having such a large amount of Mn as mentioned above. Thus, in the steel of the present invention, no recrystallization of austenite extended by rolling is caused during or after hot-rolling, but a bainite ferrite is formed from crystal grain boundaries of unrecrystallized austenite. Consequently, the distance (range) in which crystal grain can grow becomes very short, whereby a very fine structure can be obtained.

7 As a result of the above-mentioned mechanism, the steel of the present invention is characterized by having high inner stress, and therefore much greater strength,

TABLE 1 Mechanical properties Tensile tests lmpact tests Chemical compositions (weight in percent) Yield Tensile Elongapoint strength tion vEo v'lIrs Mn Cu Ni Cr Mo Nb Al B Treatment (kg/mm) (kg/mm (percent) (kgmlcm' (C l obtifashdtilld 483' 69.5 28.8 3.? .002 ..d 50.7 72.1 27.6 2.6 52.0 82.3 25.5 3.3 23 51.8 83.0 24.5 2.2 10 53.1 75.2 27.0 0.9

.002 Hardeningtem 0.3

Other objects of the present invention will be clear from the following description and the accompanying drawings, in which:

FIG. 1 shows the relation between tensile strength and impact value at 0 C. of steels of the present invention and conventional steels as rolled; and

FIG. 2 shows the relation between tensile strength and facture transition temperature (vTrs) in 2 mm V- notch impact tests of steels of the present invention and conventional steels as rolled.

The present invention relates to a steel consisting of 0.05 to 0.08% C (percent being by weight herein), 0.1 to 1.0% Si, 2.3 to 4.0% Mn, 0.03 to 0.15% Nb and 0.01 to 0.10% Al, the rest being iron and impurities. Further, the steel of the present invention is formed by adding to such steel for the purpose of securing a further improvement of the strength or the toughness at a low temperature one or more of 0. l 0 to 0.60% Cu, 0.10 to 1.00% Ni, 0.10 to 1.00% Cr, 0.10 to 0.50% Mo, 0.02 to 0.15% V, 0.01 to 0.10% Ti, 0.01 to 0.10% Zr and 0.0005 to 0.0080 B. The steel of the present invention is particularly characterized in composition by the composite addition of Mn and Nb to a low-carbon steel with a carbon content ranging from 0.05 to 0.08 percent and in the structure thus formed being a fine and uniformly-sized upper bainite.

highly improved toughness, because of the bainite ferrite becoming very fine on account of the above mentioned mechanism.

The reasons for limiting the respective component elements in the steel of the present invention as described above are as follows:

C is contained in order to impart a strehgth to a steel. If C is less than 0.05 percent, it is difficult to obtain a tensile strength of more than 60 kg/mm, preferably more than kg/mm. But if more than 0.08% C is present, a time and uniformly sized upper bainite cannot be obtained, whereby the impact transition temperature becomes suddenly high, the impact value (vEo) is reduced, and the weldability is also reduced. Therefore, in the steel of the present invention, the carbon content is limited to a range of 0.05 to 0.08 percent.

0.10% Si is the lower limit required for making the steel. However, if Si is more than 1.00 percent, its effeet on strength is not high and low temperature toughness is deteriorated. Therefore, it is desirable that the steel contain -no more than 1.00% Si.

Mn is one of the most important additive elements in the steel of the present invention. lf Mn is less than 2.3 percent, a polygonal proeutectoid ferrite is produced in the locality of the steel, and no fine and uniformly sized upper bainite structure as characteristic of the steel of the present invention can be obtained. However, if Mn exceeds 4 percent, the weldability is deteriorated. Therefore, the upper limit is made 4.0 percent. But, Mn is added, preferably in a range of from 2.55 to 4.0 percent.

Nb, as well as Mn, is one of the most important additive elements for the steel of the present invention. The composite effect of the coexistence of Nb and Mn in a very low-carbon steel is the greatest feature of the present invention. But, if Nb is less than 0.02 percent, it has no effect. On the other hand, if more than 0.16% Nb is added, no more improvement in strength can be expected, but the toughness is rather reduced. Therefore, it is desirable to add Nb in an amount of less than 0.15%. As to the composite addition effect of Mn and Nb, a further explanation must be added. If the addition of Mn is increased up to an amount as is claimed for the steel of the present invention without a sufficient addition of Nb, the strength of the steel tends to increase, but the rate of the increaseis gradually diminished with an increase in the added amount of Mn. This is to be explained by means of the solid solution hardening phenomenon. Further, the toughness of the steel increases to some extent of the added amount of Mn, for instance, up to 1.5% Mn, but shows rather a deterioration if the addition of Mn is further increased. However, if Nb is added within the range specified in the present invention, a remarkable improvement in strength occurs by the addition of Mn in amounts above 2.3 percent. At the same time, the toughness of the steel is also improved with the increase of the amount of Mn. These phenomenon can be said to be caused by the synergetic effect of the composite addition of Mn and Nb in the very low carbon steel.

Al is added as a deoxidizing agent, and its proper amount necessary and sufficient for making a steel is 0.01 to 0.10 percent.

Cu, Ni, Cr, Mo, V, Zr, Ti and B contribute also to the improvement and stabilization of the strength or the toughness of a steel by forming fine precipitations and their intermetallic compounds, which effect can be obtained by the addition of one or more of V, Zr or Ti, in the ranges of 0.02 to 0.15% V, 0.01 to 0.10% Zr and 0.01 to 0.10% Ti. Below the said lower limits, no effect is obtained. Above the upper limits, no remarkable effect can be observed. Particularly, it is to be noted that the addition of Ti is not generally very effective to the toughness of ferrite and pearlite steels, but when Ti is added to the steel of the present invention, a further improvement of the low temperature toughness can be obtained.

As regards Cu, Ni, Cr and Mo, they are properly added for the above mentioned purpose in respective ranges, as follows: Cu in a range of 0.10 to 0.60 percent, Ni in a range of 0.10 to 1.00 percent, Cr in a range of 0. 10 to 1.00 percent and M in a range of 0. 1 0 to 0.50 percent. Further, B has a hardening effect in making a bainite structure. Particularly, it shows a striking effect by the composite addition with Al and Ti. Therefore, it may be added in a range of 0.0005 to 0.0080 percent.

As mentioned above, the composite addition of Mn and Nb is the most important condition for making the steel of the present invention. However, in order to complete the understanding of the meaning of such composite addition, the following explanation must be further added. When part of the Mn is replaced with an equivalent amount of Cr, M0 or Cr Mo on the basis of the known calculating formula multiplying factor of hardenability, there cannot be obtained a steel having such a fine, uniform upper bainite structure as is seen 5 in the steel of the present invention, and also the favorable combination of the strength and toughness cannot be obtained. Also in regard to Nb, the situation is almost the same as in the case of Mn. Even when V, which is an element comparatively similar to Nb, is used instead of Nb, no such structure and mechanical properties as in the steel of the present invention can be obtained. That is to say, in the present invention, the composite addition of Mn and Nb to a steel having a carbon range as specified as a base makes an essential l5 condition for making the steel of the present invention, whereby there can be achieved a special effect which cannot be obtained with similar elements such as Cr, Mo and V in place of Mn and Nb. Of course, the addition of these similar elements does not impair the composite effect of Mn and Nb.

1n thefollowing, the present invention shall be explained with reference to examples in order to further clarify the constituents of the present invention as above-mentioned, and essential features of the steel of the present invention.

EXAMPLE 1 Table 2 shows the strength and toughness of the steels of the present invention as compared with those of some reference steels, said steels of both kinds being so-called non-tempered high-tensile strength steels in the state as hot-rolled after being ingottcd and bloomed. Steels marked with as H, l, M, N, O and the like, are all the steels of the present invention, and the others are reference steels.

The steelsK and L are examples having no addition of Nb. When no Nb is present, such strength and toughness as of the steel of the present invention cannot be obtained, even if nearly 3% Mn is added.

I For such reasons, the steel of the present invention having Mn-Nb as basic components can be said to be a new type low-alloy high-tensile strength steel. Further, it is also evidently seen from FIGS. 1 and 2 showing the relations between the strength and the toughness of these steels as rolled (in the graphs the marks epresent steels of the present invention, the marks represent steels partly deviated from the ranges claimed for the steels of the present invention, and the marks represent conventional steels), that those other than the steels of the present invention reside in the range shown by the hatching, but that the steels of the present invention are remarkably excellent in the combination of strength and toughness. Further, M to T in the graphs are those steels containing proper amounts of Cu, Ni, Mo, Ti, V, Zr and the like added to the basic composition of the present invention, respectively, and are characterized by a further improvement of strength or toughness.

- 5 EX MPLE 2 This example shows that when the steel of the present invention is tempered, there can be obtained more favorable mechanical properties and a stabilized material.

T515163 showsmechanical properties when the steel.

1 shown in Table 2 was tempered at 550, 625 and 675 C, whereby the tensile strength was reduced, but the yield point (0.2 percent proof stress in this case) was elevated and the impact value at C was increased.

'Further, the transition temperature was reduced and in general a stabilized material could be obtained.

tures. Also, although-the above-mentioned shows only some examples of the present invention, the present invention can be properly modified within the above de- I "WEAE is claimed is:

scribed ranges. v

' Mechanical properties Chemical compositions Lwt.%)

Tensile tests Impact tests is explainedin rhea save; thepres enfi nvenwon provides a low-alloy high-tensile strength steel distinguished from any conventional high-tensile strength st- Sam- Yield Tensile ElonvEo vTrs ple C Si Mn Nb Al Others point strength gation (kgmlcm (C) g/ m) (kg/mm) G 0.04 0.64 1.98 0.10 0.02 46.4 60.8 34.2 12.8 H" 0.05 0.64 2.70 0.10 0.02 60.5 80.1 25.1. 10.5, -88 1* 0.05 0.69 2.44 0.05 0.02 43.6 77.5 28.7 13.0 -54 .l 0.10 0.63 2.40 0.05 0.02 62.3 87.5 v 24.1 1.2 1 +11 K 0.05 0.66- 2.33 0.02 33.4 59.4 34.0 2.6 L 0.05 0.34 2.96: 0% 38.5 61.4 l2 10g A Q M V 0.05 0.60 2.40 0.05 0.02 Cu 0.31 57.2 83.1 26.4 9.5 u

N 0.05 0.65 2.36 0.05 0.03 Ni 0.50 56.0 82.6" 28.1 12.0 -95 -93..- 0.0. 9. 3. 2 5. 9124. 9-9 M91124. 1 .87.; 24. -0 2 P* 0.05 0.71 2.35 0.04 0.04 Ti 0.03 48.6 72.8 s 30.0 11.1 l20 0* 0.07 0.36 2.46 0.03 0.02 Cr 0.50 64.2 88.5 24.0 5.4 -15 R 0.05 0.50 2.70 0.09 0.02 V 0.04 60.1 80.5 27.1 9.5 S* 0.06 0.40 2.53 0.06 0.02 Zr 0.06 61.3 85.0 26.1 5.6 -18 T* 0.05 0.62 2.42 v 0.05 0.02 Cr 0.42 56.2 80.6 27.6 8.2 -38 Table'3 Y Sam- Treatment Yield Tensile Elonvlio v'lrs ple point strength gation (kgm/cm) ((1) g/ s/ m) W as hot- 43.6 77.5 28.7 13.0 -54 rolled 550C 51.8 68.3 32.8 15.1 -68 tempering 1 625C 52.3 64.0 32.2 16.5 -65 tempering 675C 53.0 p 64.1. 30.8 19.2 -62 tempering claim 1, further comprising from 0.0005 to 0.0080% B. 3.7K low-fifiigh tiisfie stragmsieer'raaaear a fine and uniformly-sized upper bainite structure as rolled, consisting essentially of 0.05 to 0.08 C, 0.10 to 1.00% Si, 2.3 to 4.0% Mn, 0.03 to 0.15 Nb and 0.01 to 0.10 Al, and moreover, containing at least one element selected from the group consisting of 0.10 to 0.60 Cu, 0.10 to 1.00 Ni, 0.10 to 1.00 Cr, 0.10 to 0.50 M0, 002 to 0.15 V, 0.01 to 0.10 Ti and 0.01 to 0.10 Zr, the rest being Fe and impurities. i

- 4. The low'alloy high-tensile strength steel claimed in claim 3, further comprising from 0.0005 to 0.0080% B. 

2. The low-alloy high-tensile strength steel claimed in claim 1, further comprising from 0.0005 to 0.0080% B.
 3. A low-alloy high-tensile strength steel made of a fine and uniformly-sized upper bainite structure as rolled, consisting essentially of 0.05 to 0.08 % C, 0.10 to 1.00% Si, 2.3 to 4.0% Mn, 0.03 to 0.15 % Nb and 0.01 to 0.10 % Al, and moreover, containing at least one element selected from the group consisting of 0.10 to 0.60 % Cu, 0.10 to 1.00 % Ni, 0.10 to 1.00 % Cr, 0.10 to 0.50 % Mo, 0.02 to 0.15 % V, 0.01 to 0.10 % Ti and 0.01 to 0.10 % Zr, the rest being Fe and impurities.
 4. The low-alloy high-tensile strength steel claimed in claim 3, further comprising from 0.0005 to 0.0080% B. 